1. Field of the Invention
The present invention relates to a correction circuit which is used in a horizontal deflection circuit, so as to to correct a distortion of an image displayed on the screen of a cathode ray tube.
2. Description of the Related Art
In the prior art, a horizontal deflection beam is emitted toward the display screen of an image display device, such as a CRT monitor, by supplying a current through the horizontal deflection coil of cathode ray tube.
An example of a manner in which the current flowing through the horizontal deflection coil is controlled will be described, with reference to FIGS. 1, 2 and 3A-3E.
FIG. 1 is a circuit diagram of the basic circuit of a conventional horizontal deflection circuit, and FIG. 2 is an equivalent circuit corresponding to the circuit shown in FIG. 1.
As is shown in FIGS. 1 and 2, the conventional horizontal deflection circuit is made up of a fixedvoltage source E, a horizontal deflection coil LH, a tuning capacitor C, a booster transformer TR, a switching transistor TR, and a damper diode D. The transistor TR is applied with a pulse between the base and emitter thereof, and serves as a switch S. When the switch S is open, as viewed in the equivalent circuit shown in FIG. 2, the diode D passes a current only in the reverse direction, so that no current flows through the circuit.
When the switch S is closed at time tl shown in FIG. 3A, a current whose intensity linearly increases with time flows through the deflection coil LH. At the time, the capacitor C is charged instantly. When the switch S is opened at time t2, the current flowing through the deflection coil LH decreases while simultaneously charging the capacitor C, and becomes zero at time t3. Thereafter, electricity is discharged from the capacitor C to the deflection coil LH, and a current flowing in the reverse direction to that mentioned above passes through the deflection coil LH from time t3 to time t4.
Next, in the tuning circuit made up of the deflection coil LH and the capacitor C, the capacitor C begins to be charged in the reverse direction to that mentioned above. Due to the existence of the damper diode D, however, the voltage appearing at the terminal of the deflection coil LH becomes higher than the power source voltage at time t5, which is immediately after the the time when the maximum current flows through th deflection coil LH. Since, therefore, the diode D is applied with voltage in the forward direction, the current which has flowed through the deflection coil LH also flows through the diode D, while simultaneously charging the power source, and gradually decreases. When the current flowing through the deflection coil LH becomes zero, the switch S is closed again at time t6. With the above operation repeated, the sawtooth current I shown in FIG. 4A is made to flow through the deflection coil LH.
As is shown in FIG. 5, a horizontal deflection beam BM is emitted from a predetermined point inside a cathode ray tube B toward the screen of the tube B by causing the sawtooth current I to flow through the horizontal deflection coil LH. If the sawtooth current I flowing through the horizontal deflection coil LH is not corrected, the deflection angle .theta. per unit time of the horizontal deflection beam BM is proportional to the intensity of the sawtooth current I. Accordingly, the deflection angle .theta. per unit time is constant with respect to the entire screen. This means that the distance for which the screen is scanned with the beam per unit time varies, depending upon the portions of the screen. If it is assumed that the scanning distance in the center of the screen is l 1 and that the scanning distance in a peripheral portion of the screen is l 2, then the relation below is established, as is shown in FIG. 5, EQU l2&gt;l1
As a result of this relation, the image displayed on the cathode ray tube B is horizontally elongated in the periphery of the screen.
This problem has been conventionally solved by adding a correction circuit 2 to a horizontal deflection circuit 1, as is shown in FIG. 6. More specifically, the resonance current (FIG. 4B) produced by the horizontal deflection coil LH and capacitor C of the horizontal deflection circuit 1 is superposed on the sawtooth current I, to obtain a corrected horizontal deflection current ILH having such an "S"-shaped waveform as is shown in FIG. 4C. This corrected horizontal deflection current ILH is made to flow through the horizontal deflection coil LH, in order that scanning distance l 2 becomes equal to scanning distance l 1, for the improvement of the linearity. However, since the resonance current I is dependent on both the inductance of the horizontal deflection coil LH and the capacitance of the capacitor C, it is impossible to minutely control he resonance current I. When the corrected current ILH shown in FIG. 4C is made to flow through the horizontal deflection coil LH, the scanning rate may still be low in the center of the screen and high in the periphery of the screen. Therefore, an image on the screen may remain distorted to a certain extent.
As mentioned above, even with the use of the conventional horizontal deflection circuit, the spatial linearity is not good and cannot be improved satisfactorily when the screen is scanned with the beam in the horizontal direction. This problem is due to the reason that analog control cannot be performed with respect to the horizontal deflection current ILH, as may be understood from the following prior art publications:
U.S. Pat. No. 4,019,093; PA1 U.S. Pat. No. 4,612,481; PA1 U.S. Pat. No. 4,516,058; and PA1 U.S. Pat. No. 4,468,593.